Mosto

The mosto is the fermented wort: the agave juice that has been inoculated with yeast (or left to native airborne yeast) and converted, over hours to weeks, from a sugary liquid into a low-alcohol fermented base ready for distillation. The word covers both the in-progress ferment and the finished ferment; production logs sometimes distinguish mosto fresco (fresh, sugary, pre-fermentation) from mosto muerto (literally "dead mosto," fully fermented and ready for the still).

Chemically, a finished mosto is a dilute beer: 4 to 8 percent ABV, residual sugars, carboxylic acids, esters, fusel alcohols, and a working population of yeast cells and bacteria. The microbiology drives the eventual flavor of the distillate. Tequila fermentation typically runs 2 to 5 days with selected Saccharomyces cerevisiae strains in stainless-steel tanks; mezcal fermentation runs 5 to 14 days, sometimes longer, with native airborne yeast in open wooden vats. The difference is most of why the two spirits taste so distinct.

The mosto is the stage at which the producer chooses how clean or rustic the final spirit will be. A fast, temperature-controlled ferment with a single yeast strain yields a cleaner product; a slow, ambient ferment with a mixed microbial community yields a more complex, more idiosyncratic one. See the distillation chapter Part 4 for fermentation in production context.